In order to determine an attenuation correction in a positron emission tomography (PET) scanner, use can be made of a combination of a magnetic resonance system and a positron emission tomography system, a so-called MR-PET hybrid system. However, a problem in using such MR-PET hybrid systems is the fact that local coils, which are used to receive the magnetic resonance signals from the examination object (for example, a human body), are not visible in conventional clinical magnetic-resonance scanning techniques. However, these coils can have a significant influence on the emission data of the positron emission tomography scanner, and so the attenuation thereof must be corrected in order to obtain artifact-free and quantitative PET images.
The prior art has proposed various approaches to measure the attenuation of these objects, i.e. the local coils, or to determine the attenuation thereof directly.
By way of example, in “Towards quantitative PET/MRI: A review of MR-based attenuation correction techniques” by M. Hofmann, B. Pichler, B. Schölkopf, and T. Beyer (European Journal of Nuclear Medicine and Molecular Imaging 36 (Supplement 1), 93-104 (03 2009)), the entire contents of which are hereby incorporated herein by reference, it is proposed to scan the objects in a CT scanner and to transfer the measured transmission values to the attenuation at 511 keV. However, the CT images of the object may contain metallic artifacts. Furthermore, partial volume effects may occur and the transformation of the linear attenuation coefficients for non-tissue material to 511 keV is complicated.
In “Study of MR head and neck coils for its use in an integrated MR/PET-scanner” by Gaspar Delso, Alex Martinez, Ralph Bundschuh, Ralf Ladebeck, David Faul, and Sibylle Ziegler (J. Nucl. Med. Meeting Abstracts 2009 50:1476) and in “Attenuation Correction for MR Table and Coils for a Sequential PET/MR System” by Bin Zhang, Debashish Pal, Zhigiang Hu, Navaeep Ojha, Tiantui Guo, Gary Muswick, Chi-Hua Tung, and Jeff Kaste (IEEE MIC, 2009), the entire contents of each of which are hereby incorporated herein by reference, it is proposed to scan the objects in a PET system with an emission source. However, this acquisition takes a very long time, the resulting image is very rough, and it is not possible to distinguish between small structures.
Finally, there is the option of determining the attenuation images from design drawings by converting CAD structures into volume images, wherein the individual voxels in the volume images are provided with the physical linear attenuation coefficients. However, in general, not all parts are available as a model. Often it is only the plastic parts that are available as a model. Other parts, such as e.g. metal parts, can therefore only be modeled in a generic fashion, as a result of which the actual local structure of the linear attenuation coefficients can only be derived imprecisely and with great difficulty. Moreover, each coil can slightly deviate from the determined attenuation image as a result of production tolerances.